Tuleyome Tales: How the California delta happened

Fifteen thousand years ago, the land that became known as California had no delta and was in a very different world. Much of North America and Europe were covered by vast continental ice sheets. By then, people occupied most of the Eastern Hemisphere but few, if any, had yet reached the Americas.

Then, what is now Central California’s coastline was 26 miles west of its present location. The Farallons were then not islands but coastal headlands overlooking an open ocean dropping abruptly to great depths.

What is now the continental shelf was a vast, dry land plain bisected by an ancestral Sacramento River swollen to great volume by melting glaciers then widespread in the Sierra Nevada. It entered the Pacific south of the Farallons and flowed through the Coast Range 300 feet below present water level in deep canyons at what are now the Golden Gate and Carquinez Strait.

The climate then along the lower Sacramento was much like the present coast of southern Alaska and British Columbia, but the world was warming. The great continental ice sheets began retreating, and their melt water caused seas to rise everywhere. By 10,000 years ago, they neared the present shoreline and by 8,000 years ago had entered the Golden Gate.

People were definitely in what would become California by then, and had established villages in a broad valley just inside the outermost Coast Range ridge. Soon, however, rising seas following the ancestral Sacramento River’s channel inland completely flooded their valley and created what later arrivals would call San Francisco Bay.

Inexorably, seas pushed farther inland, flooding more valleys and creating new bays like San Pablo and Suisun until they finally stopped near the present Montezuma Hills 5,000 years ago.

There, freshwater flowing downstream from the Sierra and Cascade mountains through the Sacramento and San Joaquin rivers met seawater flowing inland through the Golden Gate. They mixed some, but the freshwater mostly flowed outward some distance in a shallow lens above the heavier salt water.

That acted as a hydraulic dam to stop most river flow at the Montezuma Hills and cause freshwater to back up and flood a vast area in the lowest part of the Central Valley.

This flooding starting just 5,000 years ago created California’s Sacramento-San Joaquin Delta. Because sea level rise was gradual, the flooded area always remained very shallow beyond the deeper river channels and became covered by tall marsh plants called tules.

Seas still slowly rose, though, and freshwater in the flooded delta area also rose just slowly enough for each new tule generation to grow on the last’s flooded remains. Eventually, the latest tule generation grew on many feet of ancestral organic remains, which became the delta’s famous peat soil.

A similar process in the same time period north of East Anglia created England’s famous Fenlands and provided the term “fen” for similar wetlands around the world. Consequently the delta is California’s largest fen and one of the largest in the world. What happened to it next is another tale.

— Glen Holstein received his Ph.D. in botany from UC Davis and is a senior scientist with Zentner and Zentner, a local biological consulting company. He is a member of the board of Tuleyome, a nonprofit organization working to protect the wild and agricultural heritage of California’s Inner Coast Range and Western Sacramento Valley.